Polycarbonates are well known thermoplastic materials which, due to their many advantageous physical and mechanical properties, find use as thermoplastic engineering materials in many commercial and industrial applications. The polycarbonates exhibit, for example, excellent properties of toughness, flexibility, impact resistance, and high heat distortion temperatures. The polycarbonates and their preparation are disclosed, for example, in U.S. Pat. Nos. 2,964,974; 2,999,835; 3,028,365; 3,334,154; 3,275,601; and 3,915,926.
However, these polycarbonates generally suffer from two disadvantages. The first disadvantage is the low critical thickness values of polycarbonates, i.e., the thickness at which a discontinuity in Izod impact values occurs. These low critical thickness values tend to limit wall thickness of molded polycarbonate to a thickness below the critical thickness. Polycarbonates exhibit notched Izod impact values which are dependent on the thickness of the polycarbonates. Thus, for example, while typical notched Izod impact values for a one-eighth inch thick polycarbonate test specimen are generally in the range of about 16 foot pounds per inch, typical notched Izod impact values for a one-fourth inch thick polycarbonate test specimen are generally in the range of about 2.5 foot pounds per inch. The high Izod values of the one-eighth inch thick polycarbonate test specimen are due to the fact that these specimens are thinner than the critical thickness of the polymer and, therefore, upon impact a hinged or ductile break occurs. The low Izod impact values of the one-fourth inch thick polycarbonate test specimens are due to the fact that these specimens exceed the critical thickness of the polymer and, therefore, upon impact a clean or brittle break occurs.
The second disadvantage of polycarbonates is that they, like most other plastics, are relatively flammable. Thus polycarbonates are generally unsuitable for applications where high temperatures and exposure to flames may be encountered. In order to render the polycarbonates suitable for high temperature or open flame environments they must be modified to be made flame retardant. One of these modifications involves the inclusion a halogenated moiety, such as halogenated diphenol, in the backbone of the carbonate polymer. These halogen containing carbonate copolymers are generally flame retardant. However, the presence of these halogen containing moieties adversely affects the critical thickness values of the polycarbonates. Thus, for example, the critical thickness of a carbonate copolymer containing 5 to 6 percent by weight bromine in the form of a halogenated diphenol is about 130-140 mils.
U.S. Pat. No. 4,043,980 discloses polycarbonate compositions obtained as the reaction products of an aromatic diol, a halogenated phenol, and a carbonic acid coreacted with an aromatic thiodiphenol, which compositions exhibit flame retardancy stated to be the result of the synergism between the sulfur and the halogen present in the compositions. However, this patent teaches the necessity of the presence of a halogen containing moiety in the polycarbonate compositions and states that the resultant flame retardancy is the result of synergism between the sulfur present in the thiodiphenol and the halogen present in the halogenated phenol.
International Application No. WO 82/00468, published Feb. 18, 1982 discloses that polycarbonate compositions can be rendered flame retardant by either admixing with the carbonate polymer a polymer based on a thiodiphenol, or incorporating into the polycarbonate backbone a thiodiphenol residue. While these compositions are quite effective and useful in most applications, they suffer from the disadvantage that relatively large amounts of thiodiphenol, typically from about 23-98 mole percent, must be employed to render said compositions flame retardant. Since thiodiphenol is relatively expensive, as compared with dihydric phenols such as bisphenol-A, its use in relatively large amounts places these flame retardant polycarbonate compositions at an economic disadvantage. Also in some applications, particularly those where the polycarbonate resin is required to exhibit properties of substantially sulfur-free bisphenols, such as bisphenol-A, or where the presence of large amounts of sulfur would be disadvantageous, such large amounts of thiodiphenol are undesirable.
It would thus be very advantageous if flame retardant polycarbonate compositions could be provided which are halogen free and which, consequently, exhibit the properties of halogen-free polycarbonates such as good thick section impact strengths. It would also be very advantageous if flame retardant polycarbonate compositions could be provided which not only exhibit the properties and characteristics of halogen-free polycarbonates, but also exhibit the properties and characteristics of substantially sulfur-free polycarbonates such as good processability and economic competitiveness.
It is, therefore, an object of the instant invention to provide polycarbonates which are flame retardant, are halogen-free, and contain relatively minor amounts of sulfur in the form of thiodiphenol residues.